These findings come as a surprise, since scientists previously believed that Callisto was relatively inactive. If Callisto has an ocean, that would make it more like another Jovian moon, Europa, which has yielded numerous hints of a subsurface ocean. Despite the tantalizing suggestion that there is an ocean layer on Callisto, the possibility that there is life in the ocean remains remote.
Callisto's cratered surface lies at the top of an ice layer, (depicted here as a whitish band), which is estimated to be about 200 kilometers (124 miles) thick. Immediately beneath the ice, the thinner blue band represents the possible ocean, whose depth must exceed 10 kilometers (6 miles), according to scientists studying data from Galileo's magnetometer. The mottled interior is composed of rock and ice.
Galileo's magnetometer, which studies magnetic fields around Jupiter and its moons, revealed that Callisto's magnetic field is variable. This may be caused by varying electrical currents flowing near Callisto's surface, in response to changes in the background magnetic field as Jupiter rotates. By studying the data, scientists have determined that the most likely place for the currents to flow would be a layer of melted ice with a high salt content.
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